Duluth-Superior harbor freshwater corrosion update

In the early 2000's, it was discovered that the steel sheet piling in the Duluth-Superior Harbor (DSH) was corroding at a rate much greater than observed in other Great Lakes fresh water harbors and ports. Much of the port's over 13 miles of steel sheet pile walls are heavily pitted, with several facilities having severe perforations entirely through the steel sheets. Holes as large as 10 inches or more in diameter have now been discovered. Once the Duluth-Superior accelerated corrosion had been discovered and reported, other Great Lakes ports and harbors also started to observe steel corrosion issues in some structures, although not as severe as in the DSH. Since the fall of 2004, a multi-partner project steering committee has provided a detailed, systematic focus for the needed research and mitigation recommendations to determine the cause and extent of the corrosion, as well as explore potential methods for the repair of existing damaged steel sheets. Funding was secured for several focused studies to answer why the accelerated corrosion is occurring, and determine potential options to slow or eliminate corrosion on existing and new steel structures. This paper will 1) update the extent of accelerated fresh water corrosion observed in both the DSH and other Great Lakes harbors, 2) discuss the results of completed and on-going studies to determine the cause(s) of the accelerated harbor corrosion, and 3) explore methods being tested for steel structure mitigation and protection. Preliminary test results indicate that iron-oxidizing bacteria are present on the corroded structures, and testing continues to determine whether these or other bacteria may be causing the accelerated corrosion. Protective coating trials have shown promising results for protecting steel structures. Testing continues to explore how different coatings withstand the severe ice impact and abrasion observed in the DSH and other cold region environments during winter conditions. Several of the alternative corrosion control product tests have also shown promising results for extending the usable lives of existing structures.